Low-pressure laminate, often called “low-pressure board”, is a well known, industrially important, and moderate to low cost product used in many industries including the furniture industry. In general, it is formed by a “low pressure” laminating procedure using a suitable substrate and a decorative paper facing sheet, i.e. a paper decor sheet, which may be a solid color or have a design, e.g. a wood grain design, printed on its surface and which is impregnated with a thermosettable resin such as melamine-formaldehyde resin, often simply called “melamine resin”, some other amino resin such as urea-formaldehyde resin, or an unsaturated polyester resin, and optionally with a similar resin impregnated barrier sheet interposed between the substrate and the decor sheet.
The substrate can be formed of a variety of materials, such as thermosettable resin impregnated paper sheets, but more usually plywood, chipboard, fiberboard such as MASONITE® fiberboard, particleboard, wafer board or the like. Examples of such low pressure laminates are described in O'Dell et al U.S. Pat. No. 5,422,168, the entire contents of which are hereby incorporated by reference.
High pressure decorative laminates are laminates which meet a number of critical industry standards promulgated by NEMA, i.e. NEMA standards. These laminates are and have for many years been conventionally produced by stacking and curing under heat and pressure a plurality of layers of paper impregnated with various synthetic thermosetting resins. In normal practice, the assembly from the bottom up consists of a plurality, e.g. three to eight, core sheets made from phenolic resin impregnated Kraft paper, above which lies a decor sheet impregnated with melamine resin. A protective overlay sheet is often provided on top of the decor sheet. This overlay sheet, hereinafter simply “overlay”, is almost transparent in the laminate and provides protection for the decor sheet.
However, it is cumbersome and unduly costly to use overlay in the manufacture of a low pressure laminate, wherein the low cost of the product is important, which means that many print designs are unfit for use in a low pressure laminate due to poor abrasion resistance.
In both high pressure and low pressure laminates, the decor sheet may be a high quality, 50–125 lbs. ream weight (81.5 to 203.75 g/m2), pigment filled paper that has been impregnated with a water-alcohol solution of melamine resin, dried and partially cured, and finally cut into sheets. As indicated above, the decor sheet, prior to impregnation with the resin, may have been printed with a decorative design, or with a rotogravure or inkjet printed reproduction of natural materials, such as wood, marble, leather, etc. Alternatively, the decor sheet is solid colored. Conventionally, ink is used to produce the printed design on the decor sheet. In recent years, the trend in the printing industry has been to replace organic solvent based inks with water based inks.
Examples of high pressure decorative laminates are found in, among others, Scher et al U.S. Pat. No. 4,255,480; Ungar et al U.S. Pat. No. 4,713,138; Ungar et al U.S. Pat. No. 5,037,694; O'Dell et al U.S. Pat. No. 4,499,137; O'Dell et al U.S. Pat. No. 4,532,170; O'Dell et al U.S. Pat. No. 4,567,087; O'Dell et al U.S. Pat. No. 5,344,704; and O'Dell et al U.S. Pat. No. 5,545,476, the entire contents of which are hereby incorporated by reference.
A protective coating, such as NEVAMAR ARP® and/or “Armored Protection Plus” as per at least some of the above noted patents, often eliminates the need for overlay to protect the printed surface of the high pressure laminate. Elimination of the overlay improves visual clarity of the appearance of the decor sheet. In the ARP® and/or “Armored Protection Plus” technologies, the surface layer which protects the decor sheet from abrasion is an overcoating which is greatly reduced in thickness, compared to overlay, so as to provide a highly concentrated layer of abrasion resistant particles or other protective particles bound to the upper surface of the uppermost paper layer, usually the decor sheet.
As noted above, in many high pressure decorative laminate products meeting NEMA standards and having a printed surface, the printed surface is protected by the overlay. In some of these decorative laminates, a protective coating such as the aforementioned ARP® eliminates the need for an overlay, such that the printed surface is very close to the uppermost surface of the laminate, making a quality bond between the ink and the paper a critical parameter for product performance. The ARP® and “Armored protection plus” technologies have served the industry exceedingly well, and high pressure decorative laminate incorporating ARP® and/or “Armored Protection Plus” usually well exceed NEMA abrasion resistance standards. Unfortunately, however, in the absence of an overlay, some surface printed designs, especially those based on aqueous ink systems, show unacceptable, premature wear, even when protected by such a protective coating.
Mordants are well known compounds of various types which are commonly used to bond dyes to textile fibers, e.g. by linking to both the dye molecule and the fiber molecule. Mordants are particularly used with dyes, called “mordant dyes” or “lake pigments”, which have little or no substantively or affinity for textile fibers. “Mordant” and “mordant dye” are defined in “Grant & Hackh's Chemical Dictionary”, 5th edition (1987) as follows:                Mordant A chemical used for fixing colors on textiles by absorption; as, soluble salts of aluminum, chromium, iron, tin, antimony m.dye An artificial or natural color for fibers which usually forms an insoluble metal compound (lake) with metallic salts (mordant).        
The Condensed Chemical Dictionary, 9th edition, defines these terms as follows:                Mordant A substance capable of binding a dye to a textile fiber. The mordant forms an insoluble lake (q.v.) in the fiber, the color depending on the metal of the mordant. The most important mordants are trivalent chromium complexes, metallic hydroxides, tannic acid, etc. Mordants are used with acid dyes, basic dyes, direct dyes, and sulfur dyes. Premetalized dyes contain chromium in the dye molecule. A mordant dye is a dye requiring use of a mordant to be effective. See also dye, fiber-reactive.        
Such mordant dyes are applied to cellulosic or protein fibers that have been pre-treated (mordanted), usually with metallic oxides, to give points of attraction of the later applied dye. The dye forms a complex with the mordant and, depending upon the particular metal and fiber, can form a large molecule which is less capable of desorbing from the fiber, or can form a dye molecule bound to the fiber resulting from chelation with the metal. Some effective dyes result from introducing metals such as chromium and cobalt into dye molecules to produce larger molecules; these complexes can be formed by chelating one or two molecules of a dye with the metal.
The most commonly used mordants for natural dyes are alum (potassium aluminum sulfate), chrome (potassium dichromate or potassium bichromate), blue vitriol (copper sulfate), ferrous sulfate, stannous chloride, sodium dithionite or sodium hydrosulfite, ammonium hydroxide, cream of tartar (potassium bitartrate), “glauber's salt” (sodium sulfate), lime (calcium oxide), lye (sodium hydroxide), oxalic acid, tannic acid, urea, vinegar (acetic acid), and washing soda (sodium carbonate). Other mordants used include salts of iron, copper, tin, and other heavy metals. Still other mordants include citric acid or mixtures of an aluminum salt, citric acid, and a carbonate, such as disclosed in Gurley et al U.S. Pat. No. 5,651,795, the entire contents of which are hereby incorporated by reference.